4 This yields a prediction for the static structure factor in the low-momentum limit: K () (p)! p! 1:84::: p 4 9 (4.6) which is in good agreement with the results of the nite size analysis [17]. For =14 p 13=14, we x u(p) in such a way that the ratio (3.6) of the moments with n = 1 and n = is reproduced correctly. u(p) is shown in gure 5(a). It turns out to be smaller than 1 for p > =3 and larger than 1 for p < =3. The extrapolation to p = seems to meet the value given in (4.5). Finally, we determine the p-dependence of the amplitude A(p) from the moment sum rule (3.8) with n = 1. A(p) is shown in gure 5(b). The extrapolation to p = is compatible with the behaviour given in (4.5). The values of A(p) and u(p) at p = : A() = 1:88(8); u() = :63(5); (4.7) have been obtained in reference [17] from a t to the static structure factor of the form: K () (p)! p! A() 1 ln p + const: (4.8) 2 In gure 4(b) we compare the estimate for (t; p) with the prediction MM (t; p), which follows from the ansatz (4.3) with the momentum dependent amplitude A(p) and the modied high-frequency cut-o (4.4). The dierence: MM (t; p) = MM (t; p) (t; p) (4.9) is obviously much smaller than (4.1), which is shown in gure 4(a). On the other hand (4.9) is not yet zero. In particular for large t and p = 3=4 we still observe sizeable deviations form our estimate (t; p) for the thermodynamical limit, leaving room for further improvements of the ansatz (4.3). 5. Conclusions The euclidean time representation (1.8) is particularly suited for a study of nite size eects in dynamical structure factors. In this paper we investigated the spin- 1=2 antiferromagnetic Heisenberg model (1.1). We found that nite size eects in the euclidean time representation die out with N 2 for those momentum values p where there is a gap (1.4) between the groundstate and the rst excited state with momentum p. We then compared our estimate for the thermodynamical limit with two predictions. Each of them follows from an ansatz of the type (4.3) for the spectral (!)- representation of the dynamical structure factor: The rst ansatz { i.e. (4.3) with momentum independent amplitude A(p) = A and u(p) = 1 { has been proposed by Muller et al. [12]. It takes into account two-spinon contributions only and respects the properties (i) and (ii) on the frequency moments

5 8 The error in our estimate of (t; p) is presumably less than the width of the lines in gure 3. This optimistic view appears to be justied by the clean signal for the nite size behaviour (3.9) seen in the data. 4. A modied ansatz for the dynamical structure factors In gure 4(a) we compare our estimate for (t; p) with the prediction M (t; p), which follows from the ansatz (1.6). The dierence: M (t; p) = M (t; p) (t; p) (4.1) is negative for p < =3 and positive for p > =3. It increases with t and p for p > =3. The increase of the deviations with p has been observed also in the static structure factors [17]. Before presenting modications of the ansatz, which are in better agreement with the estimate of the thermodynamical limit let us briey review the properties of the frequency moments K (n) (p) { dened in (3.7): (i) The moment (3.8) with n = 1 is determined by the groundstate energy. (ii) The higher moments with n odd are known [13] to be polynomials of order n in cos p. (iii) The moment with n = 1 is related to the static susceptibility (p), which yields in the zero momentum limit [12]: K ( 1) (p)! () p! = : (4.2) 2 (iv) The moment with n = is identical with the static structure factor, which has been determined in reference [17] from a nite size analysis in the momentum interval =14 p 13=14. The ansatz of Muller et al. [12] respects the rst two properties. However, the rst and third one cannot be satised simultaneously with this ansatz. We therefore would like to propose a modication here: S(!; p) = q A(p) (!! 1 )(^! 2!) (4.3)! 2! 2 1 with a momentum dependent amplitude A(p) and a modied high-frequency cut-o: ^! 2 (p) u(p)! 2 (p): (4.4) Properties (i) and (iii) are satised now if we choose: u(p)! p! 1:274:::; A(p)! p! :7475::: p: (4.5)

8 Table 1. Comparison of leading excitation energies and transition probabilities with the results of complete diagonalization [15] on a ring with 16 sites. 5 S( =, p = /4) = S( =, p = /2) = !n(=4) wn(=4)!n(=2) wn(=2) O(1 4 ) 6. O(1 4 ) S( =, p = 3/4) = S( =, p = ) = !n(3=4) wn(3=4)!n() wn() O(1 7 ) the recursion method with ~ L = 4 iterations is given in table 2. Again we only list those digits which agree with the numerically exact result. The latter is obtained from a determination of the groundstate in the channels with total spin S = and S = 1, respectively. 3. Finite size analysis of the dynamical structure factors in the euclidean time representation For noncritical momenta p < and euclidean times not too large nite size eects are small in the dynamical structure factors (1.8). In gure 1 we show the momentum

9 4 with matrix elements: hf k jojf k i = 8 >< >: 1 : k = k + 1 a k : k = k b 2 k : k = k 1 : else (2.6) in the basis jf k i. The eigenvalues of O yield the excitation energies! = E n E and the eigenvectors represent the excited states jni of (H E ) in the basis jf k i: jni = LX k= jf k i hf kjni hf k jf k i : (2.7) The zero components hf jni of the eigenvectors and the excitation energies E n E determine the dynamical structure factor: S(; p; N) = X n jhf jnij 2 exp[ (E n E )]: (2.8) Performing the iteration (2.2) numerically, one nds that the orthogonality of the vectors jf k i is lost after a certain number of steps due to rounding errors. On the other hand it turns out that the energies E n E and transition probabilities of the low-lying excitations with n < 1 can be obtained already with high accuracy by truncating the problem. We stop the iteration after L ~ steps and diagonalize the truncated L ~ L ~ matrix O. As an example we show in table 1 the rst 9 excitation energies and normalized transition probabilities w n (p; N) = jhnjs 3(p)jij 2 S(; p; N) (2.9) on a ring with N = 16 sites and momenta p= = 1 ; 1 ; 3 ; The approximate solution given in table 1 is obtaind by truncating the iteration after L ~ = 4 steps. We only list in table 1 those digits which agree with the numerically exact solution given in [15]. In other words, the number of quoted digits measures the accuracy of the approximation. Note that the energies and transition probabilities of the rst four excitations are reproduced correctly with 8 and more digits. The accuracy for the remaining 5 excitations is less impressive. However, this inaccuracy has practically no eect on our evaluation of the dynamical structure factors in the euclidean time representation (1.8), since the contributions of the higher excitations are suppressed twofold. In addition to the exponential damping factor exp(!) in (1.8) it turns out, that the transition probabilities themselves drop rapidly with!. Therefore we expect, that the determination of the excitation energies and transition probabilities by means of the recursion method will yield as well reliable results for larger systems with N = 18; 2; :::; 28. In these systems we have checked the energy and the transition probability of the rst excitation with momentum p =. The result obtained with

10 in!. In order to extract the thermodynamical limit it is useful [15] to consider the Laplace transform of (1.2): S(; p; N) = Z 1! 1 d! S(!; p; N) exp(!) (1.8) = hjs + 3 (p) exp[ (H E )]s 3 (p)ji: It can be interpreted as a euclidean time ()-representation of the dynamical structure factor. In this paper we want to propose a new method to compute the dynamical structure factors. It is based on the recursion used in [16] as input for the continued fraction approach. In section 2 we will demonstrate, how the energies and transition probabilities for the low excitations can be obtained directly from the recursion approach. In section 3 we will present our results on the dynamical structure factors (1.8) for systems with N = 4; 6; 8; :::; 28. Finite size eects will be analyzed and an estimate for the thermodynamical limit will be given. This estimate is compared in section 4 with the prediction (1.6) of Muller et al.. We also propose a modication of this ansatz, which yields better agreement with the estimate of the dynamical structure factors in the thermodynamical limit The Recursion method Following [16], (1.8) can be computed by iteration. For this purpose one expands the euclidean time evolution of the 'initial' state jf i s 3 (p)ji: exp[ (H E )]s 3 (p)ji = 1X k= D k ()jf k i (2.1) in terms of an orthogonal basis jf k i which is constructed recursively by application of the Hamilton operator: (H E )jf k i = jf k+1 i + a k jf k i + b 2 kjf k 1 i; k = ; 1; 2; ::: (2.2) a k = hf kjh E jf k i ; k = ; 1; 2; ::: (2.3) b 2 k = hf k jf k i hf kjf k i hf k 1 jf k 1 i k = 1; 2; :::; (2.4) with jf 1 i and b 2. On nite systems the iteration (2.2) will terminate after L steps, where L is the dimension of the Hilbert space spanned by the states (H E ) l s 3 (p)ji, l = ; 1; :::; L 1. The iteration generates a tridiagonal L L-matrix O: (H E )jfi = Ojfi (2.5)

11 2 1. Introduction Quantum spin systems with known dynamical behaviour are rare. The spin 1=2 XX-model can be mapped on a free fermion system [1] and the dynamical spin-spin correlators can be computed analytically [2-5]. A second example is the Haldane- Shastry model [6,7] { an isotropic Heisenberg model with couplings which decrease with the inverse square of the distance between two spin operators. In this model only two-spinon excited states contribute to the zero temperature dynamical structure factor [8,9]. In this paper we are concerned with the familiar antiferromagnetic Heisenberg model with nearest neighbour coupling: H = 2 NX x=1 ~s(x)~s(x + 1) (1.1) and periodic boundary conditions. The dynamical structure factors at T = : S(!; p; N) = X n (! (E n E ))jhnjs 3 (p)jij 2 (1.2) are dened by the transition probabilities jhnjs 3 (p)jij 2 from the groundstate ji to the excited state jni with energies E and E n, respectively. The transition operator s 3 (p) = p 1 N NX x=1 exp(ipx)s 3 (x) (1.3) is just the Fouriertransform of the spin-operator s 3 (x) at site x. The lower bound for the excitation energies! 1 =! 1 (p) sin p (1.4) has been computed by Cloizeaux and Pearson [1]. The infrared behaviour at p = : S(!; p = ; N = 1) ;!!! 1 (1.5) has been investigated in reference [11]. G. Muller and collaborators [12] proposed an ansatz, which takes into account only two spinon excitations: S(!; p) = A (!! 1)(! 2!) q : (1.6)! 2! 2 1 The upper bound of these excitations! 2 (p) 2 sin p 2 (1.7) has been obtained from the Bethe ansatz. The ansatz (1.6) has been shown [13] to satisfy certain conditions on the spectral moments for the dynamical structure factor. Moreover, it was successfully applied to the description of neutron scattering data [14]. On nite systems, the dynamical structure factors are dened as -function contributions

12 Computation of dynamical structure factors with the recursion method A Fledderjohann, M Karbach, K-H Mutter y and P Wielath Physics Department, University of Wuppertal 4297 Wuppertal, Germany Abstract. We compute the energies and transition probabilities for low excitations in the one dimensional antiferromagnetic spin-1/2 Heisenberg model by means of the recursion method. We analyse nite size eects in the euclidian time ()- representation and compare the resulting estimate for the thermodynamical limit with two parametrizations for the dynamical structure factors in the spectral (!)- representation. PACS numbers: b Short title: Computation of dynamical structure factors July 11, 1995 y

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